In the past, for soldering in various kinds of electric and electronic devices, much use has been made of lead-tin (Pb—Sn) based solder alloys from the viewpoint of their low melting point, good wettability even in the air or other oxidizing atmospheres, etc. On the other hand, since Pb has toxicity, regulations have conventionally been imposed on operations using Pb or Pb-containing alloys etc. The frequency of occurrence of Pb poisoning etc. has heretofore been kept extremely low.
Due to the recent rising increase in interest in environmental protection, however, the social trend is to require measures to be taken for the disposal of various devices using Pb-containing solder alloys, in particular electric and electronic devices.
Up until now, used electronic devices have mainly generally been disposed of by dumping at landfills in the same way as ordinary industrial waste or general waste. However, if continuing to dispose of used electronic devices using large amounts of Pb-containing alloys by dumping at landfills, there is a concern that the elution of Pb will have a harmful effect on the environment and life.
Therefore, in the future, it will probably be made compulsory to dispose of used electronic devices using large amounts of Pb-containing solder alloy only after recovering the Pb.
However, up to now, no technology has been established for efficiently and effectively removing the Pb from used electronic devices. Further, the recovery costs of the Pb are liable to cause an increase in the cost of the products.
Therefore, development of a Pb-free solder alloy not containing any Pb is strongly desired.
Up unto now, as a Pb-free solder alloy, for example an alloy based on Sn and containing in combination Zn (zinc), Ag (silver), Bi (bismuth), Cu (copper), and the like has been put into limited practical use, but this has been limited to special applications. This is because the various characteristics required in general applications which have used Pb—Sn solder alloys in the past, that is, a low melting point and good wettability, capability for reflow treatment, freedom from reaction with the base material to cause formation of a brittle compound layer or an embrittled layer, and other characteristics (solderability) have not been obtained.
At the present time, a Sn—Zn solder alloy is being proposed as a promising Pb-free solder alloy. A Sn—Zn solder alloy has a melting point of near 200° C. and may very well be able to replace a conventional Sn—Pb solder alloy.
However, Zn easily oxidizes and is poor in solder wettability, so it is necessary to use nitrogen gas or another non-oxidizing atmosphere in order to ensure good solderability.
To improve the solder wettability of a Sn—Zn solder alloy, it is proposed to add Cu (copper) or Ge (germanium), but the anticipated improvement in the wettability is not obtained. Instead, the addition of C causes rapid formation of Cu—Zn intermetallic compounds in the solder alloy, so has the defect of deterioration of the properties of the solder alloy.
Moreover, Zn has an extremely high activity. When soldering on a Cu base material, a thick layer of Cu—Zn intermetallic compounds ends up easily forming even with a small heat input and becomes a cause of reduction of the bond strength. The base material/solder interface structure in this case presumably has a structure of a Cu base material/β′-CuZn layer/γ-Cu5Zn8 layer/solder layer. A Cu—Zn intermetallic compound has a very weak bond strength at the interface with the solder, so exfoliation easily occurs. A similar phenomenon ends up occurring even when the surface of a Cu base material is plated with Ni (nickel)/Au (gold), plated with palladium, and plated with palladium/gold. Therefore, from the viewpoint of the reliability of the electronic device, practical use of a Sn—Zn solder alloy has been difficult.